Lightspeed
Counter-examples Some outlets have reported experiments that give results countering the proposition that the speed of light in a vacuum is a constant. |ScienceNews://Speed of light not so constant after all> - January 17, 2015 "Light doesn’t always travel at the speed of light. A new experiment reveals that focusing or manipulating the structure of light pulses reduces their speed, even in vacuum conditions. A paper reporting the research, posted online at arXiv.org and accepted for publication, describes hard experimental evidence that the speed of light, one of the most important constants in physics, should be thought of as a limit rather than an invariable rate for light zipping through a vacuum." "The researchers produced pairs of photons and sent them on different paths toward a detector. One photon zipped straight through a fiber. The other photon went through a pair of devices that manipulated the structure of the light and then switched it back. Had structure not mattered, the two photons would have arrived at the same time. But that didn’t happen. Measurements revealed that the structured light consistently arrived several micrometers late per meter of distance traveled." |arXiv://Photons that travel in free space slower than the speed of light> - 14 Nov 2014 "That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wavevectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam's transverse spatial structure. Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several microns over a propagation distance of the order of 1 m. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount." Responses Responses have clarified why such examples actually support the proposition, rather than refute it: |arXiv://Comment on “Spatially structured photons that travel in free space slower than the speed of light”> "D. Giovannini et al. (Reports, 20 February 2015, p. 857) reported that they measured spatially structured photons travelling in free space slowing down even in vacuum. Here we present a simple quantum mechanical consideration which shows that even in these cases photons travel with the speed of light ©, and this measurement provided experimental results on the “projection” of this velocity to the axis of symmetry/beam propagation." "According to our interpretation the measurement experimentally confirms that the component z of the momentum of a single photon in a Bessel beam state possess a definite value simultaneously with the magnitude of the momentum. Generally, the magnitude of the momentum and the energy of the quantum of the field always have definite values in states obeying the Helmholtz equation. Consequently, the photon travels with a velocity that exactly equals the speed of light. For Gaussian and other types of beams the experiment above measures the expectation value of the component z of the photon momentum." Category:Relativity Category:General Relativity Category:Light Category:Physics